A 11-year-old, spayed, female mixed breed-dog was presented with an abdominal mass that was detected 1 month ago. Upon abdominal palpation a large, firm, oval shaped, movable mass was found in the mid-abdominal region. Survey radiograph of the abdomen demonstrated an oval soft tissue dense mass located on the right side of the abdominal cavity. A large, heteregenous and cystic mass with solid components occupying the majority of the abdomen and a small, cystic mass with solid components caudal to the left kidney were identified by transabdominal ultrasonography. Computed tomography scans revealed bilateral ovarian masses, and a small volume of retroperitoneal fluid on the right side. A cystic, but otherwise solid mass located in the right ovary and small retained left ovary encapsulated in the ovarian bursa were excised surgically by midline laparotomy. Histopathological examination of the excised mass from the right side revealed a large cystic structure consistent with an ovarian cyst and multiple corpora lutea and follicles at different maturational stages were detected in the left ovary. The precise origin of the ovarian cyst could not be determined by morphological appearance. Immunohistochemical staining suggested a cyst of surface epithelial origin. At re-examination 6 months after the surgery, the bitch appeared healthy and the clinical findings were all normal. To our knowledge, the cyst described here is the largest reported in an incompletely ovariohysterectomized bitch.